1. Field of the Invention
The present invention generally relates to a nonvolatile ferroelectric memory device, and more specifically, to a technology of controlling read/write operations of a nonvolatile ferroelectric memory cell using a channel resistance of a memory cell which is differentiated by polarization states of a ferroelectric material in a nano scale memory device.
2. Description of the Related Art
Generally, a ferroelectric random access memory (hereinafter, referred to as ‘FeRAM’) has attracted considerable attention as next generation memory device because it has a data processing speed as fast as a Dynamic Random Access Memory (hereinafter, referred to as ‘DRAM’) and conserves data even after the power is turned off.
The FeRAM having structures similar to the DRAM includes the capacitors made of a ferroelectric substance, so that it utilizes the characteristic of a high residual polarization of the ferroelectric substance in which data is not deleted even after an electric field is eliminated.
The technical contents on the above FeRAM are disclosed in the Korean Patent Application No. 2001-57275 by the same inventor of the present invention. Therefore, the basic structure and the operation on the FeRAM are not described herein.
A unit cell of a conventional nonvolatile FeRAM device comprises a switching element and a nonvolatile ferroelectric capacitor. The switching element performs a switching operation depending on a state of a word line to connect a nonvolatile ferroelectric capacitor to a sub bit line. The nonvolatile ferroelectric capacitor is connected between and a plate line and one terminal of the switching element.
Here, the switching element of the conventional FeRAM is a NMOS transistor whose switching operation is controlled by a gate control signal.
FIG. 1 is a cross-sectional diagram illustrating a conventional nonvolatile ferroelectric memory device.
A conventional 1-T (One-Transistor) FET (Field Effect Transistor) cell comprises a N-type drain region 2 and a N-type source region3 which are formed on a P-type region substrate 1. A ferroelectric layer 4 is formed on a channel region, and a word line 5 is formed on the ferroelectric layer 4.
The above-described conventional nonvolatile FeRAM device reads and writes data by using a channel resistance of the memory cell which is differentiated depending on polarization states of the ferroelectric layer 4. That is, when the polarity of the ferroelectric layer 4 induces positive charges to the channel, the memory cell becomes at a high resistance state to be turned off. On the contrary, when the polarity of the ferroelectric layer 4 induces negative charges to the channel, the memory cell becomes at a low resistance state to be turned on.
However, in the conventional nonvolatile FeRAM device, when the cell size becomes smaller, a data maintaining characteristic is degraded, so that it is difficult to perform the normal operation of the cell. That is, a voltage is applied to an adjacent cell at read/write modes to destroy data of unselected cells, so that interface noise is generated between the cells and it is difficult to perform a random access operation.